NASA scientists have come up with a surprisingly simple yet effective way to deflect an Earth-bound asteroid  park a large spacecraft close by and let gravity do the work.

Previous suggestions have focused on deflecting an incoming asteroid with nuclear explosions. But NASA experts believe a "gravity tractor" should be able to perform the same feat by creating an invisible towline to tug the rock off its deadly course.

"Most people think of the Hollywood treatment  throw a nuclear weapon at it," says Edward Lu, a NASA scientist and astronaut who developed the idea. But this would produce shattered pieces, some of which might still head towards Earth. "Thats the blast-and-hope strategy," Lu adds.

Pots of paint

Another proposal is to detonate nuclear bombs close to the asteroid. The resulting blast of radiation should nudge it off track but there is the same risk of wayward fragments if the asteroid shatters.

A more novel idea is to paint the surface of the asteroid white. This should change the amount of solar energy it radiates and change its course. However, the amount of paint required could be huge.

Lu and colleagues originally thought about landing a spacecraft on the surface of an asteroid, in order to gently push it off course. But a lack of gravity means the craft would have to attach itself to the surface of the rock and this could prove complicated because the asteroid might be little more than a pile of rubble, Lu explains.

To make matters worse, asteroids often rotate, so pushing on one could simply set it spinning faster, rather than altering its course.

Hands down winner

Lus team finally realised that the spacecraft might not need to land at all. Placing a heavy enough object near the asteroid for long enough could produce sufficient gravitational tug to change its orbit.

For a 200-metre-wide asteroid, the spacecraft would need to weigh about 20 tonnes and lurk 50 metres from its target for about a year to change its velocity enough to knock it off course.

"This is hands down the best idea I have seen," says Erik Asphaug, a planetary scientist at the University of California at Santa Cruz. "This will work, but you need to put a large enough spacecraft out there at the right time."

Taking the hit

Such large spacecraft are perfectly feasible, says Lu. In fact, NASAs multi-billion-dollar Prometheus programme, which was set to explore the outer solar system but which has been delayed, planned to develop just such a vehicle, propelled by nuclear fission.

The strategy crucially relies on our ability to detect an asteroid threat about 20 years in advance. For larger asteroids this is realistic. But Asphaug says many smaller asteroids  less than about 500 metres across  may go unnoticed until only a few years before impact.

Asphaug suggests it may be better to invest in predicting when and where smaller asteroids could strike, than on massive hazard-averting spacecraft. Governments could then prepare to evacuate affected regions. "In many cases it makes more economic sense to just let the thing hit," he says